Materials called gradient functional materials have hitherto been known and are being enthusiastically developed. For example, Japanese Patent Laid-Open No. 345977/1993 discloses a structure of the material and an apparatus for producing it, and “gradient functional material” is defined therein as “a material in which the internal compositional distribution changes continuously”. Effects expected of such materials in which the compositional distribution changes continuously include the enhancement of heat resistance, and many developments have been achieved from this standpoint. In particular, in the field of space environment, such gradient functional materials are attracting attention as materials capable of withstanding extreme temperature changes.
On the other hand, in the field of electronics also, the effectiveness of a material whose compositional distribution changes with a gradient in the formation of an insulating layer is disclosed in Japanese Patent Laid-open No. 18316/1993.
In the field of optics also, gradient functional materials are receiving attention. Japanese Patent Laid-Open No. 231248/1996 discloses a technique in which a material whose refractive index changes with a gradient is formed as an antireflection film on a glass substrate or the like. An example proposed therein is a structure which comprises a glass substrate having a refractive index of about 1.5 and a thin film formed on a surface of the substrate. In this structure, the refractive index of the thin film is about 2.0 for a part thereof close to the substrate, gradually decreases with reducing distance to the uppermost surface of the film, and becomes about 1.55 at the uppermost surface.
The reference teaches that a glass surface which has a reflectance of about 4% when it has no thin film formed thereon can be made to have a reflectance of 1.5% or lower by forming such a thin film thereon. A technique heretofore in general use for realizing such an antireflection film is to alternately deposit a high-refractive-index material and a low-refractive-index material. The method disclosed therein is expected to be a technique which can more easily realize an antireflection function in a wide wavelength range.
As described above, gradient functional materials are materials which are attracting attention in many fields. With respect to applications, however, most of these fall under the category in which the property attributable to a gradient in compositional distribution is utilized.
On the other hand, the field of microprocessing technology also has achieved a remarkable development. In particular, semiconductor processing techniques have greatly progressed and made it possible to conduct processing on the order of 1 μm or less. The main technique is photolithography. The technology of microprocessing is extensively used not only for semiconductors but in the field of optics. In particular, lenses having a diameter of 1 mm or smaller, which are far smaller than the lenses heretofore in use, diffraction gratings having alternating ridges and grooves of about 1 μm, and the like are produced with techniques of microprocessing. The field in which such minute optical elements are dealt with is recently often called microoptics. General production processes and all optical elements obtained thereby are roughly explained in Microoptics Technology, written by N. F. Borrelli (published by Maecel Dekker, Inc. in 1999).
Examples of methods for producing optical elements include plastic molding, glass molding, and the like. Besides these, a combination of photolithography and the technique of reactive etching is frequently used in producing minute lenses, diffraction gratings, diffraction lenses, or the like. For obtaining particular optical properties by such a method, three-dimensional convexity (lens) or grooves (optical diffraction element) are necessary. A method of chemically etching (wet etching) a material in a solution, which is a technique most easily usable, is disclosed in Japanese Patent Laid-Open No. 123771/1999. In this method, a stamper for a microlense array is formed through etching. When this technique, which is for forming spherical concave parts, is used for etching an isotropic material such as a glass, a shape having spherical surfaces attributable to the isotropy of etching is obtained.
In the case of materials which are not isotropic in etching, such as crystals, a shape attributable to the symmetry of crystals can be realized through etching. Typical examples of anisotropic etching include the formation of a V-shaped groove using a single crystal of silicon. In this case, a groove is formed in a specific direction so as to have a shape having a specific angle, and this groove is utilized, e.g., as a fixing groove for optical fibers.
However, in those homogeneous materials shown as examples, etching characteristics depend on properties inherent in these materials regardless of whether they are isotropic or anisotropic, and there have been limitations in realizing various desired shapes. In the case of lenses, for example, formation of an aspheric lens is essentially impossible. In the case of diffraction gratings having V-shaped grooves, freedom of utilization can be secured by changing the angle of the grooves in a wide range. However, the etching, which depends on properties of the materials, only gives grooves which are limited in angle, and the freedom thereof is hence significantly limited.
An aim of the invention, which has been achieved in order to eliminate such problems, is to considerably relax limitations on the shapes obtained through etching, by using a gradient material.